The transmission of high definition television signals dramatically increases the amount of image information within the broadcast signal over that present in conventional television broadcast formats such as the NTSC format used in the United States of America. In view of the limited number of television broadcast channels available and the generally crowded condition of available broadcast spectra, it is advantageous to transmit high definition television signals in a so-called spectrum compatible format. Spectrum compatible requires that high definition television signals be capable of transmission within a standard six megahertz channel bandwidth used in the NTSC format. Because the image information required for high definition television transmission greatly exceeds that which may be fitted within a standard six megahertz bandwidth, practitioners in the art employ digital video information coding and sophisticated data compression techniques. While a number of types of data compression systems have been devised, generally all make use of the tendency of the image information between successive image frames to include significant redundancy.
In one type of data compression system, often referred to as motion compensated predictive coding, temporal redundancies are removed from the transmitted signal using a process which divides each image frame into a plurality of image or pixel blocks. The process then compares the video pixels in each block within the current image frame to the pixels within each block of the previous image frame. These comparisons are typically carried forward using means square error or absolute error computation to identify the most closely matching set of image blocks in the current frame and previous frame. The change in position of the matching block within the current frame from that of the corresponding image block in the previous frame is used to generate a so-called motion vector. The motion vectors associated with each image block are then transmitted and used within the high definition television receiver to read blocks of stored pixels representing the previous frame and reconstruct the blocks of the current frame.
Several problems are inherent in such data compression techniques which arise largely from the purely mathematical algorithm used to identify matching pixel blocks and generate the motion vectors. One manifestation of such problems arises from the possibility that the image block matching operation may, in essence, incorrectly match a current frame image block and previous frame image block due to the comparison being based upon the smallest error or difference between image blocks. This may result in substantial noise in the reproduced block and may also adversely affect the error masking operation carried forward within the receiver.
In anticipation of the reception of erroneous or contaminated data due to transmission difficulties and the like, the high definition television receiver utilizes an error masking system to minimize the degradation of the displayed image when incorrect or contaminated data is presented. In essence, such error masking systems utilize the image information received prior to the detected errors or data contamination. In the operation of motion compensated predictive coding data compression operations, the error masking system usually utilizes the previous motion vector to reconstruct the current image frame. This process, of course, is based upon the assumption that the video image is characterized by continuous motion and thus the previous motion vector will generate a block of pixels close to the desired block. In some instances, however, such as scene changes or the like, this assumption proves to be invalid and the error masking process will fail.
Another problem associated with data compression systems using motion compensated predictive coding techniques is their susceptibility to high frequency noise. In essence, high frequency noise may unduly influence or, in some instances, dominate the pixel block matching process. This again raises the possibility for incorrectly matching pixel image blocks within the current and previous frames and, as a result, generating erroneous motion vectors.
There remains, therefore, a continuing need in the art for evermore improved data compression systems which function reliably in the presence of noise or other image discontinuities such as scene changes, special effects or the like.